Magnetic recording disks are a popular and important form of storing large quantities of information (so-called "mass storage") for computers. Relative to other forms of mass storage, they are reliable, easy to manufacture, capable of storing large amounts of data, and allow rapid storage and retrieval of information. They consist of a disk, typically of a metal such as aluminum, on which is deposited a coating of magnetic material. Conceptually, the disk is divided into concentric circles ("tracks"). Radial lines divide the disk into pie shaped "sectors". Information is stored on the disk by applying a local magnetic field to a portion of a given track in a given sector. The magnetic material at that location stores the applied magnetic field. The information can later be read by examining where the stored magnetic field changes direction. The locations at which the magnetic field changes direction are known as "transtions".
One method of categorizing magnetic recording disks is whether they are "longitudinal" or "vertical". Vertical disks are sometimes referred to as "perpendicular".
In a longitudinal disk, the magnetic fields of the coating are aligned in a predominantly circumferential direction. Transitions occur when the magnetic field changes from clockwise to counterclockwise, and vice versa. At certain locations on the disk, the magnetic field may have both a radial and a circumferential component, but only the circumferential component is read.
In a vertical disk, the magnetic field is in a direction along an axis perpendicular to the plane of the disk. Transitions occur when the magnetic field changes from upward to downward, and vice versa.
It is desirable to store as much information as possible on each disk. One method of doing so is to make the tracks as narrow and as close together as possible. However doing so can increase the problem of "noise". One kind of noise is caused when reading to or writing from a location interferes with the data that is stored in a nearby location, or when reading information at nearby locations is mistakenly read as the data stored at the intended location.
Another important characteristic of magnetic recording disks is signal strength. The greater signal strength a magnetic field has, the less the likelihood that "noise" will cause erroneous readings. Unfortunately, increasing the track density decreases the signal strength.
Thus, as track densities increase, it is important to find ways to reduce noise. One method of doing so is to introduce a second layer of magnetic material between the substrate and the magnetic coating that stores the data. This second layer of magnetic material is of a material that does not retain its magnetic field when the current which induces it is removed. Such materials are often referred to as "soft" magnetic layers". Materials that maintain their magnetic field when the current that induces them is removed are referred to as "hard", and layers of such materials are referred to as "hard magnetic layers". The thickness of a hard magnetic layer is dependent on factors such as the distance between the disk and the read/write head (the "fly height), and the density of the recording. In a longitudinal disk, the hard magnetic layer is typically on the order of 2 micro-inches thick, and the soft magnetic layer, if included, is approximately the same thickness as the hard layer. In a vertical disk, the hard magnetic layer is typically on the order of 4 to 20 micro-inches thick. The thickness of the soft magnetic layer is dependent on the thickness of the write pole, and is typically 16 to 20 micro-inches thick.
Unfortunately, a soft magnetic layer has the property of decreasing the signal strength of a longitudinal disk. This property can be alleviated, however, by placing a layer of magnetically inert material between the soft magnetic layer and the hard magnetic layer. The thickness of the magnetically inert layer is dependent on the fly height. A typical thickness of an inert layer is 5 to 10 micro-inches.
Other aspects of soft magnetic layers in longitudinal recording are described in U.S. patent application No. 07/103,965, entitled Magnetic Medium for Longitudinal Recording, filed Oct. 5, 1987 by Mallary, et al, and assigned to the assignee of the present application. In Mallary, the uniformity of the pulse shape and the minimization of soft layer induced noise is enhanced by a radial orientation of the easy axis of magnetization of the soft magnetic layer.
In a vertical disk the soft magnetic layer has the property of increasing the signal strength. This is a highly desirable property, because, as information density increases, signal strength tends to decrease.
The beneficial properties of a soft magnetic layer can be enhanced by orienting the magnetic material such that their magnetic field is oriented in one direction, thus establishing a direction in which the aggregate magnetic field of all the material is stronger; the property of having one direction in which the magnetic field is stronger is called "uniaxial anisotropy". The direction in which the magnetic field is stronger is often referred to as the "easy axis", or the "uniaxial anisotropic axis". The axes perpendicular to the "easy axis" are referred to as "hard axes". In a magnetic recording disk it is common to ignore the hard axis that is perpendicular to the plane of the disk, and to use the term "hard axis to refer to the axis coplanar with the disk and perpendicular to the easy axis.
In one embodiment of the magnetic recording disk medium using the vertical magnetic recording method, there is a so-called two-layer film medium consisting of a vertical hard magnetic layer made from CoCr alloy, an anodized or NiP plated aluminum base, and in between, a soft magnetic layer made from Permalloy. The recording and reproduction characteristics of this two-layer medium are strongly dependent on the magnetic characteristics of the soft magnetic layer.
In general, the permeability of the soft magnetic layer along the hard axis is higher than the permeability of the soft magnetic layer along the easy axis; therefore, in order to obtain good recording and reproduction characteristics, it is necessary to align the hard axis of the soft magnetic layer to the direction of the tracks (circumferential direction) of the magnetic recording disk. A method for doing this is disclosed in Japanese Kokai Patent No. Sho 62[1987]-129940. As shown in FIG. 1, an inner magnet placed in the center of the uncoated disk and an outer magnet is placed at the outer edge of the disk. The poles of the magnets are aligned such that the lines of the magnetic field are in a radial direction from the center of the disk base. When the soft magnetic layer is deposited, typically by sputtering, the magnetic field causes the magnetic material to align such that the easy axis is in a radial direction, and the hard axis is in a circumferential direction.
However, when applying this method using the magnets described above for the production of the magnetic recording disk medium using in-line sputtering apparatus, the magnets have to be very thin. As a result, it is difficult to obtain a magnetic field, in the radial direction, at the surface of the disk base with sufficient intensity (approximately 10 oersted) normally required for this process.
Moreover, although it is sufficient to fix the outer magnet surrounding the outer circumference of the disk base, on the pallet (base holder), it is necessary to attach the inner magnet to the base itself in order for the soft magnetic layers to formed simultaneously on both surfaces of the disk base. But this is undesirable due to the increase in the number of processing steps and the forming of dust during the mounting and dismounting of the magnet.
If this process is used during the formation of a vertical disk, there is an additional problem. During the forming of the vertical magnetized layer that follows the forming of the soft magnetic layer, the vertical magnetic anisotropy of the vertical magnetic layer tends to worsen due to the magnetic field, whose lines of force extend radially outward from the central magnet, in the direction of the surface of the disk base.